It makes me seem petty (or pettier), but "Thank you" isn't even enough.
I'm become pretty freakin' stingy with critique online, even here. If I spend hours going over someone's manuscript noting mistakes, weaknesses, questions, and such, and even throw in lessons on lay-lie, punctuation with direct addresses, and format, all couched in a written critique rich with humor and encouragement, I feel like I've earned more than a two-work acknowledgment. I want indication that the recipient values the time, effort, and expertise, such as it is, that went into the critique.
Don't get me wrong--I've received that sort of response many times here. But the times when my four hours is not acknowledged at all, or merits only two words, or is countered with defensiveness or hostility, makes me loath to critique again for months.
In recent times, I tend to critique online only at AW and only for people who are not new here and whose paths have crossed mine enough that I have at least some feel for who they are. A new arrival doesn't get that big a chunk of my time, not even if s/he PMs me asking.
Maryn, selfish be-yatch